This invention relates to the synthesis of dithioesters which can be utilized in a free radical polymerization process with characteristics of a living polymerization. The diothioesters may be monomeric, oligomeric or polymeric. The invention also relates to processes for the synthesis of polymers utilising these dithioesters.
There is increasing interest in methods for producing a variety of polymers with control of the major variables affecting polymer properties. Living polymerizations provide the maximum degree of control for the synthesis of polymers with predictable well defined structures. The characteristics of a living polymerization are discussed by Quirk and Lee (Polymer International 27, 359 (1992)) who give the following experimentally observable criteria:
xe2x80x9c1. Polymerization proceeds until all of the monomer has been consumed. Further addition of monomer results in continued polymerization.
2. The number average molecular weight (or the number average degree of polymerization) is a linear function of conversion.
3. The number of polymer molecules (and active centers) is a constant which is sensibly independent of conversion.
4. The molecular weight can be controlled by the stoichiometry of the reaction.
5. Narrow molecular weight distribution polymers are produced.
6. Block copolymers can be prepared by sequential monomer addition.
7. Chain end-functionalised polymers can be prepared in quantitative yield.xe2x80x9d
Thus living polymerization processes can be used to produce polymers of narrow molecular weight distribution containing one or more monomer sequences whose length and composition are controlled by the stoichiometry of the reaction and the degree of conversion. Homopolymers, random copolymers and/or block polymers may be produced with a high degree of control and with low polydispersity.
Syntheses of certain polymers with xanthate or dithiocarbamate end groups from the corresponding disulfides have been described in the literature (see for example, Moad and Solomon xe2x80x9cThe Chemistry of Free Radical Polymerizationxe2x80x9d, Pergamon, London, 1995, pp 337-339; Nair and Clouet J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1991, C31, 311). Processes have also been described that use these compounds to prepare block copolymers. However, the processes using these compounds are unsuccessful in producing low polydispersity polymers and do not meet many of the criteria for living polymerization as defined above.
WO 98/01478, the entire contents of which is incorporated herein by reference, describes the use of chain transfer agents (CTAs) of the following structure: 
in free radical polymerisation processes with living characteristics to provide polymers of controlled molecular weight and low polydispersity.
It has now been found that CTAs of this structure can be prepared in a convenient manner from a single disulphide reagent, or generated in situ in the polymerisation vessel.
Accordingly the present invention provides a process for the synthesis of dithioesters formula I: 
where R is derived from a free radical R. and is selected from the group consisting of optionally substituted alkyl; optionally substituted, saturated, unsaturated or aromatic carbocyclic rings; optionally substituted, saturated, unsaturated or aromatic heterocyclic rings; optionally substituted alkylthio, optionally substituted arylthio; optionally substituted alkoxy; optionally substituted dialkylamino; organometallic species; and polymer chains; R. is a free radical leaving group;
U is independently selected from the group consisting of hydrogen; halogen; and C1-4 alkyl optionally substituted with one or more substituents independently selected from the group consisting of hydroxy, carboxy, acyloxy, ORxe2x80x3, O2CRxe2x80x3 and CO2Rxe2x80x3;
V is independently selected from the group consisting of hydrogen, halogen, Rxe2x80x3, CO2H, CO2Rxe2x80x3, CORxe2x80x3, CN, CONH2, CONHRxe2x80x3, CONR2xe2x80x3, O2CRxe2x80x3 and ORxe2x80x3;
Z is selected from the group consisting of hydrogen, chlorine, optionally substituted alkyl, optionally substituted aryl, optionally substituted alkylthio, optionally substituted alkoxycarbonyl, xe2x80x94CO2Rxe2x80x3, xe2x80x94CO2H, xe2x80x94O2CRxe2x80x3, xe2x80x94CONR2xe2x80x3, cyano, xe2x80x94[P(xe2x95x90O)(ORxe2x80x32], and xe2x80x94[P(xe2x95x90O)Rxe2x80x32];
Rxe2x80x3 is independently selected from the group consisting of optionally substituted C1-18 alkyl, optionally substituted C2-18 alkenyl, optionally substituted aryl, optionally substituted heterocyclyl, optionally substituted aralkyl, optionally substituted alkaryl, wherein said optional substituents are selected from the group consisting of epoxy, alkoxycarbonyl, aryloxycarbonyl, isocyanato, cyano, silyl, halo and dialkylamino;
and n is 0 or a positive integer,
which process includes contacting a disulphide of the Formula II 
with
(i) in the case of n=0, a free radical of the formula R., or
(ii) in the case of n greater than 0, a free radical of the formula R. and at least one monomer of the formula III 
R may be selected from any organic group which is derived from the corresponding radical, R., which radical can act as a free radical leaving group. Preferably the radical, R., is capable of initiating free radical polymerisation. Examples of suitable radicals include carbon centred, sulphur centred, and in some circumstances, oxygen or nitrogen centred radicals. Where R is a polymer chain it may be produced by means of free radical polymerisation, or any other means, such as condensation polymerisation.
The term xe2x80x9cfree radical leaving groupxe2x80x9d as used herein refers to a group which departs as a free radical during a substitution or displacement reaction.
R may also be derived from a dithioester of formula I generated in situ, or may be derived from an initiating radical or a propagating radical.
Preferably Z is selected to give the Cxe2x95x90S bond a high reactivity towards radical addition while not slowing the subsequent rate of fragmentation in the presence of the monomer to the extent that there is an unacceptable retardation of polymerization.
When conducted in the presence of a monomer of formula III the process according to the invention is useful for the preparation of a wide variety of polymer types, including homopolymers, copolymers and block copolymers. Homopolymers may be prepared by using a single monomer of formula III, while copolymers may be prepared by using two or more monomers. Block copolymers may be prepared by contacting a first monomer of formula III with the disulphide of formula II and free radical of formula R. to produce an intermediate homopolymer of formula I, and then contacting the homopolymer with a second monomer of formula III and a free radical of formula R. Further blocks can be add in like manner.
The invention also relates to the used of a disulphide of formula II to provide chain transfer in a free radical polymerization process, and to the use of the disulphide in the preparation of a chain transfer agent of formula I for use in a free radical polymerisation process.
Free radical polymerizations in the presence of chain transfer agents (CTAs) represented by the following structure has been described in Le et al. Int. Patent Appl. WO 98/01478. 
Such polymerizations possess the characteristics of a living polymerization in that they are capable of producing polymers of pre-determined molecular weight with narrow molecular weight distribution (low polydispersity), and, by successively adding different monomers, can be used to make block polymers. The process can also be used to produce polymers of more complex architecture, including variously branched homo- and copolymers.
It has now been found that certain materials which are not CTAs of the above structure may nonetheless be utilized as precursor materials to said CTAs and with suitable choice of reaction conditions can be used in a xe2x80x98one-potxe2x80x99 synthesis of CTA and narrow polydispersity polymer.
In particular it has now been found that compounds of Formula II react with free radicals produced from a radical source (e.g. an azo compound) to form CTAs of Formula I in moderate to high yields. 
The reaction may optionally be carried out in the presence of a monomer. The initial product may, in this case, be an oligomeric CTA of Formula I. 
When the compound of Formula II is largely consumed polymerization can proceed according to the process disclosed in WO98/01478. It has been found that the overall procedure can be used to synthesise narrow polydispersity and block polymers without the need for product isolation. The process may also be adapted to the synthesis of polymers of more complex architecture through appropriate choice of compound of Formula II and monomers.
The source of free radicals should chosen such that
(i) there is a minimum level of initiator derived by-products.
(ii) the derived group R conveys appropriate reactivity to the product CTA of Formula I.
(iii) the radical R. is capable of adding to the desired monomer(s) (so as initiate subsequent polymerization steps).
The requirements for R groups in compounds of Formula I have been described in detail by Le et al. Int. Patent Appl. WO 98/01478.
As used herein the term xe2x80x9cdithioesterxe2x80x9d refers to a compound, which may be monomeric, oligomeric of polymeric having a xe2x80x94(Cxe2x95x90S)Sxe2x80x94 moiety.
Unless specified otherwise the term xe2x80x9calkylxe2x80x9d used either alone or in compound words such as xe2x80x9calkenyloxyalkylxe2x80x9d and xe2x80x9calkylthioxe2x80x9d denotes straight chain or branched alkyl, preferably C1-20alkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl, isoamyl, sec-amyl, 1,2-dimethylpropyl, 1,1-dimethyl-propyl, hexyl, 4-methylpentyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2,-trimethylpropyl, 1,1,2-trimethylpropyl, heptyl, 5-methoxyhexyl, 1-methylhexyl, 2-ethylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1,2-dimethylpentyl, 1,3-dimethylpentyl, 1,4-dimethyl-pentyl, 1,2,3,-trimethylbutyl, 1,1,2-trimethylbutyl, 1,1,3-trimethylbutyl, octyl, 6-methylheptyl, 1-methylheptyl, 1,1,3,3-tetramethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-methyl-octyl, 1-, 2-, 3-, 4- or 5-ethylheptyl, 1-, 2- or 3-propylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8-methylnonyl, 1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4- or 5-propylocytyl, 1-, 2- or 3-butylheptyl, 1-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-methylundecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8-ethyldecyl, 1-, 2-, 3-, 4-, 5- or 6-propylnonyl, 1-, 2-, 3- or 4-butyloctyl, 1,2-pentylheptyl and the like.
The term xe2x80x9calkoxyxe2x80x9d denotes straight chain or branched alkoxy, preferably C1-20 alkoxy. Examples of alkoxy include methoxy, ethoxy, n-propoxy, isopropoxy and the different butoxy isomers.
The term xe2x80x9calkenylxe2x80x9d denotes groups formed from straight chain or branched alkenes including ethylenically mono-, di- or poly-unsaturated alkyl groups as previously defined, preferably C2-20 alkenyl. Examples of alkenyl include vinyl, allyl, 1-methylvinyl, butenyl, iso-butenyl, 3-methyl-2-butenyl, 1-pentenyl, 1-hexenyl, 3-hexenyl, 1-heptenyl, 3-heptenyl, 1-octenyl, 1-nonenyl, 2-nonenyl, 3-nonenyl, 1-decenyl, 3-decenyl, 1,3-butadienyl, 1-4,pentadienyl, 1,3-hexadienyl and 1,4-hexadienyl
The term xe2x80x9cacylxe2x80x9d either alone or in compound words such as xe2x80x9cacyloxyxe2x80x9d, xe2x80x9cacylthioxe2x80x9d, xe2x80x9cacylaminoxe2x80x9d or xe2x80x9cdiacylaminoxe2x80x9d denotes carbamoyl, aliphatic acyl group and acyl group containing an aromatic ring, which is referred to as aromatic acyl, or a heterocyclic ring which is referred to as heterocyclic acyl, preferably C1-20 acyl. Examples of acyl include carbamoyl; straight chain or branched alkanoyl such as formyl, acetyl, propanoyl, butanoyl, 2-methylpropanoyl, pentanoyl, 2,2-dimethylpropanoyl, hexanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, undecanoyl, dodecanoyl, tridecanoyl, tetradecanoyl, pentadecanoyl, hexadecanoyl, heptadecanoyl, octadecanoyl, nonadecanoyl and icosanoyl; alkoxycarbonyl such as methoxycarbonyl, ethoxycarbonyl, t-butoxycarbonyl, t-pentyloxycarbonyl and heptyloxycarbonyl; cycloalkylcarbonyl such as cyclopropylcarbonyl, cyclobutylcarbonyl, cyclopentylcarbonyl and cyclohexylcarbonyl; alkylsulfonyl such as methylsulfonyl and ethylsulfonyl; alkoxysulfonyl such as methoxysulfonyl and ethoxysulfonyl; aroyl such as benzoyl, toluoyl and naphthoyl; aralkanoyl such as phenylalkanoyl (e.g. phenylacetyl, phenylpropanoyl, phenylbutanoyl, phenylisobutylyl, phenylpentanoyl and phenylhexanoyl) and naphthylalkanoyl (e.g. naphthylacetyl, naphthylpropanoyl and naphthylbutanoyl; aralkenoyl such as phenylalkenoyl (e.g. phenylpropenoyl, phenylbutenoyl, phenylmethacryloyl, phenylpentenoyl and phenylhexenoyl and naphthylalkenoyl (e.g. naphthylpropenoyl, naphthylbutenoyl and naphthylpentenoyl); aralkoxycarbonyl such as phenylalkoxycarbonyl (e.g. benzyloxycarbonyl); aryloxycarbonyl such as phenoxycarbonyl and napthyloxycarbonyl; aryloxyalkanoyl such as phenoxyacetyl and phenoxypropionyl; arylcarbamoyl such as phenylcarbamoyl; arylthiocarbamoyl such as phenylthiocarbamoyl; arylglyoxyloyl such as phenylglyoxyloyl and naphthylglyoxyloyl; arylsulfonyl such as phenylsulfonyl and napthylsulfonyl; heterocycliccarbonyl; heterocyclicalkanoyl such as thienylacetyl, thienylpropanoyl, thienylbutanoyl, thienylpentanoyl, thienylhexanoyl, thiazolylacetyl, thiadiazolylacetyl and tetrazolylacetyl; heterocyclicalkenoyl such as heterocyclicpropenoyl, heterocyclicbutenoyl, heterocyclicpentenoyl and heterocyclichexenoyl; and heterocyclicglyoxyloyl such as thiazolylglyoxyloyl and thienylglyoxyloyl.
Saturated, unsaturated or aromatic carbocyclic or heterocyclic rings may contain from 3 to 20 carbon atoms.
The terms xe2x80x9caromatic carbocyclic ringxe2x80x9d and xe2x80x9caromatic heterocyclic ringxe2x80x9d as used herein refer to aromatic and pseudoaromatic rings which may be carbocyclic or heterocyclic, and may be mono- or polycyclic ring systems. In the case of heterocyclic rings, there will be one or more heteroatoms selected from N, S, O and P. Examples of suitable rings include but are not limited to benzene, biphenyl, terphenyl, quaterphenyl, naphthalene, tetrahydronaphthalene, 1-benzylnaphthalene, anthracene, dihydroanthracene, benzanthracene, dibenzanthracene, phenanthracene, perylene, pyridine, 4-phenylpyridine, 3-phenylpyridine, thiophene, benzothiophene, naphthothiophene, thianthrene, furan, pyrene, isobenzofuran, chromene, xanthene, phenoxathiin, pyrrole, imidazole, pyrazole, pyrazine, pyrimidine, pyridazine, indole, indolizine, isoindole, purine, quinoline, isoquinoline, phthalazine, quinoxaline, quinazoline, pteridine, carbazole, carboline, phenanthridine, acridine, phenanthroline, phenazine, isothiazole, isooxazole, phenoxazine and the like, each of which may be optionally substituted. The term xe2x80x9caromatic ring compound(s)xe2x80x9d includes molecules, and macromolecules, such as polymers, copolymers and dendrimers which include or consist of one or more aromatic or pseudoaromatic rings. The term xe2x80x9cpseudoaromaticxe2x80x9d refers to a ring system which is not strictly aromatic, but which is stabilized by means of delocalization of xcfx80 electrons and behaves in a similar manner to aromatic rings. Examples of pseudoaromatic rings include but are not limited to furan, thiophene, pyrrole and the like.
The terms xe2x80x9csaturated carbocyclic ringxe2x80x9d and xe2x80x9csaturated heterocyclic ringxe2x80x9d as used herein refer to carbocyclic and heterocyclic rings, which may be mono or polycyclic ring systems, and which are fully saturated. In the case of heterocyclic rings, there will be one or more heteroatoms selected from N, S, O and P. Examples of suitable rings include, but are not limited to cyclobutane, cyclopentane, cyclohexane, cyclopentane, imidazolidene, pyrazolidene and the like.
The terms xe2x80x9cunsaturated carbocyclic ringxe2x80x9d and xe2x80x9cunsaturated heterocyclic ringxe2x80x9d as used herein refer to carbocyclic and heterocyclic rings, which may be mono or polycyclic, which have one or more degrees of unsaturation. In the case of heterocyclic rings, there will be one or more heteroatoms selected from N, S, O and P. Examples of suitable rings include cyclopentene, cyclohexene, imidazoline and pyrazoline.
Initiating radicals are free radicals that are derived from the initiator or other species which add monomer to produce propagating radicals. Propagating radicals are radical species that have added one or more monomer units and are capable of adding further monomer units.
The term xe2x80x9cpolymerxe2x80x9d as used herein includes oligomers and in the case of dithioesters of formula I refers to those in which n greater than 1. The upper limit of a xe2x80x9cnxe2x80x9d will be defined by the particular conditions and reactants employed, as well as the characteristics of the growing polymer chain.
All of the benefits which derive from the use of radical polymerization can now be realized in synthesis of low polydispersity homo- and copolymers. The ability to synthesize block, graft, star, gradient and end-functional polymers further extends the value of the process as does compatibility with protic monomers and solvents.
The source of initiating radicals can be any suitable method of generating free radicals such as the thermally induced homolytic scission of a suitable compound(s) (thermal initiators such as peroxides, peroxyesters, or azo compounds), the spontaneous generation from monomer (e.g., styrene), redox initiating systems, photochemical initiating systems or high energy radiation such as electron beam, X- or gamma-radiation. The initiating system is chosen such that under the reaction conditions there is no substantial adverse interaction of the initiator or the initiating radicals with the transfer agent under the conditions of the experiment. The initiator should also have the requisite solubility in the reaction medium or monomer mixture.
Suitable sources of free radicals for the process carried out in the absence of monomer are those which generate carbon-centred radicals. These include azo compounds and certain peroxides such as:
2,2xe2x80x2-azobis(isobutyronitrile), 2,2xe2x80x2-azobis(2-cyano-2-butane), dimethyl 2,2xe2x80x2-azobis(methyl isobutyrate), 4,4xe2x80x2-azobis(4-cyanopentanoic acid), 4,4xe2x80x2-azobis(4-cyanopentan-1-ol), 1,1xe2x80x2-azobis(cyclohexanecarbonitrile), 2-(t-butylazo)-2-cyanopropane, 2,2xe2x80x2-azobis[2-methyl-(N)-(1,1)-bis(hydroxymethyl)-2-hydroxyethyl] propionamide, 2,2xe2x80x2-azobis[2-methyl-N-hydroxyethyl)]-propionamide, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramidine)dihydrochloride, 2,2xe2x80x2-azobis(2-amidinopropane)dihydrochloride, 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramine), 2,2xe2x80x2-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide), 2,2xe2x80x2-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide), 2,2xe2x80x2-azobis[2-methyl-N-hydroxyethyl)propionamide], 2,2xe2x80x2-azobis(isobutyramide)dihydrate, 2,2xe2x80x2-azobis(2,2,4-trimethylpentane), 2,2xe2x80x2-azobis(2-methylpropane), dilauroyl peroxide, tertiary amyl peroxides and tertiary amyl peroxydicarbonates.
For reactions carried out in the presence of monomer where the properties of R. are determined by the nature of the propagating radical formed by addition to monomer a wider range of free radical sources may be used. These include sources of oxygen centred radicals such as the following initiators:
t-butyl peroxyacetate, t-butyl peroxybenzoate, t-butyl peroxyoctoate, t-butyl peroxyneodecanoate, t-butylperoxy isobutyrate, t-amyl peroxypivalate, t-butyl peroxypivalate, di-isopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, dicumyl peroxide, dibenzoyl peroxide, potassium peroxydisulfate, ammonium peroxydisulfate, di-t-butyl hyponitrite, dicumyl hyponitrite.
Photochemical initiator systems are chosen to have the requisite solubility in the reaction medium or monomer mixture and have an appropriate quantum yield for radical production under the conditions of the polymerization. Examples include benzoin derivatives, benzophenone, acylphosphine oxides, and photo-redox systems.
Redox initiator systems are chosen to have the requisite solubility in the reaction medium or monomer mixture and have an appropriate rate of radical production under the conditions of the polymerization, there initiating systems can include combinations of the following oxidants and reductants:
oxidants: potassium peroxydisulfate, hydrogen peroxide, t-butyl hydroperoxide.
reductants: iron (II), titanium (III), potassium thiosulfite, potassium bisulfite.
Other suitable initiating systems are described in recent texts. See, for example, Moad and Solomon xe2x80x9cThe Chemistry of Free radical Polymerizationxe2x80x9d, Pergamon, London, 1995, pp53-95.
Compounds suitable as monomers or comonomers include the following:
methyl methacrylate, ethyl methacrylate, propyl methacrylate (all isomers), butyl methacrylate (all isomers), 2-ethylhexyl methacrylate, isobornyl methacrylate, methacrylic acid, benzyl methacrylate, phenyl methacrylate, methacrylonitrile, alpha-methylstyrene, methyl acrylate, ethyl acrylate, propyl acrylate (all isomers), butyl acrylate (all isomers), 2-ethylhexyl acrylate, isobornyl acrylate, acrylic acid, benzyl acrylate, phenyl acrylate, acrylonitrile, styrene, functional methacrylates, acrylates and styrenes selected from glycidyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate (all isomers), hydroxybutyl methacrylate (all isomers), N,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, triethyleneglycol methacrylate, itaconic anhydride, itaconic acid, glycidyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate (all isomers), hydroxybutyl acrylate (all isomers), N,N-dimethylaminoethyl acrylate, N,N-diethylaminoethyl acrylate, triethyleneglycol acrylate, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide, N-tert-butylmethylacrylamide, N-n-butylmethylacrylamide, N-methylolmethylacrylamide, N-ethylolmethacrylamide, N-tert-butylacrylamide, N-n-butylacrylamide, N-methylolacrylamide, N-ethylolacrylamide, vinyl benzoic acid (all isomers), diethylaminostyrene (all isomers), alpha-methylvinyl benzoic acid (all isomers), diethylamino alpha-methylstyrene (all isomers), n-vinylbenzene sulfonic acid, p-vinylbenzene sulfonic sodium salt, trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, tributoxysilylpropyl methacrylate, dimethoxymethylsilylpropyl methacrylate, diethoxymethyl-silylpropylmethacrylate, dibutoxymethylsilylpropyl methacrylate, diisopropoxymethylsilylpropyl methacrylate, dimethoxysilylpropyl methacrylate, diethoxysilylpropyl methacrylate, dibutoxysilylpropyl methacrylate, diisopropoxysilylpropyl methacrylate, trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, tributoxysilylpropyl acrylate, dimethoxymethylsilylpropyl acrylate, diethoxymethylsilylpropyl acrylate, dibutoxymethylsilylpropyl acrylate, diisopropoxymethylsilylpropyl acrylate, dimethoxysilylpropyl acrylate, diethoxysilylpropyl acrylate, dibutoxysilylpropyl acrylate, diisopropoxysilylpropyl acrylate, vinyl acetate, vinyl butyrate, vinyl benzoate, vinyl chloride, vinyl fluoride, vinyl bromide, maleic anhydride, N-phenylmaleimide, N-butylmaleimide, N-vinylpyrrolidone, N-vinylcarbazole, butadiene, isoprene, chloroprene, ethylene and propylene.
Particularly preferred monomers include styrenic and methacrylate monomers.
Unless otherwise specified the term xe2x80x9coptionally substitutedxe2x80x9d as used herein means that the compound, moiety or atom may be substituted with one or more substituents selected from alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, alkoxy, alkenyloxy, aryloxy, benzyloxy, haloalkoxy, haloalkenyloxy, haloaryloxy, isocyano, cyano, formyl, carboxyl, dialkylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acyloxy, alkylsulphonyloxy, arylsulphenyloxy, heterocyclyl, heterocycloxy, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy mercapto, alkylthio, benzylthio, acylthio, sulphonamido, sulfanyl, sulfo and phosphorus-containing groups, alkoxysilyl, silyl, alkylsilyl, alkylalkoxysilyl, phenoxysilyl, alkylphenoxysilyl, alkoxyphenoxy silyl and arylphenoxy silyl.
These substituents do not take part in the polymerization reactions but form part of the terminal groups of the polymer chains and may be capable of subsequent chemical reaction. The low polydispersity polymer containing any such reactive group is thereby able to undergo further chemical transformation, such as being joined with another polymer chain. Suitable reactive substituents include: epoxy, hydroxy, alkoxy, acyl, acyloxy, carboxy (and salts), sulfonic acid (and salts), alkylcarbonyloxy, isocyanato, cyano, silyl, halo, and dialkylamino. Alternatively, the substituents may be non-reactive such as alkoxy, alkyl or aryl. Reactive groups should be chosen such that there is no adverse reaction with the CTA under the conditions of the experiment. For example, groups such as primary or secondary amino under some conditions may react with dithioesters to give thioamides thus destroying the CTA.
The process of this invention offers a route of compounds of Formula I that may be otherwise difficult to prepare.
One application is the synthesis of functional dithioesters from an azo compound containing the appropriate functionality as illustrated in the following Scheme. 
When conducted in the presence of monomers the process also offers a route to narrow polydispersity and block polymers and may be adapted to produce polymers of more complex architecture.
Whilst not wishing to be limited to any particular mechanism, it is believed that the mechanism of the process is as summarized in Scheme 2 below. Radicals (R.) are formed from a source of free radicals. These react with the compound 2 to form a compound 1 and a thiocarbonylthio radical 3. 
The thiocarbonylthio radical 3 is a poor initiator of polymerization and under the reaction conditions most likely consumed by coupling with another free radical. This radical may be a thiocarbonylthio radical to reform compound 2 or an initiator derived radical (R.) to form further compound 1.
The main side reactions occurring under the reaction conditions are most likely the self reaction of species R. and other reactions involving the source of free radicals.
For reactions carried out in the presence of a monomer the radical R. may undertake one or more monomer additions before reaction with 1 or 3. In this case the product will be of formula I with n greater than 0.
It has also been found that other compounds and processes which generate dithioester compounds of Formula I may be used in similar fashion and thus also form part of this invention. Compounds of this type are represented by formulae V and VI. 
Accordingly in another aspect of the present invention there is provided a process for the synthesis of a dithioester of Formula I from a compound of Formula V: 
where Z is as defined above,
X is hydrogen, halogen, acyl, aryl or phosphine oxide, and
each R is the same or different and is selected from the group consisting of optionally substituted alkyl; optionally substituted, saturated, unsaturated or aromatic carbocyclic rings; optionally substituted, saturated, unsaturated or aromatic heterocyclic rings; optionally substituted alkylthio, optionally substituted arylthio; optionally substituted alkoxy; optionally substituted dialkylamino; organometallic species; and polymer chains; and where the corresponding radical, R., is a free radical leaving group; by
A) in the case of n=0,
(i) where X is not hydrogen, homolytic cleavage of the Cxe2x80x94X bond of a compound of formula V and subsequent elimination of R.,
(ii) where X is hydrogen, contacting a compound of formula V with a hydrogen abstracting radical followed by elimination of R., or
(iii) contacting a compound of formula V with a Lewis acid followed by elimination of R., or
B) in the case of n greater than 0,
performing any of (i), (ii) or (iii) in the presence of a monomer of formula III as described above.
The homolytic cleavage may be achieved by, for example, thermal or photochemical means.
As shown below, homolytic cleavage of the Cxe2x80x94X bond of 5 will generate radical 7 which can fragment to give a compound of 1. 
Similarly hydrogen atom abstraction from compound 8 will generate radical 7. 
Examples of compound of Formula V are thioacetals of benzoin and related species. These compounds undergo photochemical homolytic scission. 
In yet another aspect there is provided a process for the synthesis of a dithioester of formula I as defined in claim 1 including contacting a compound of formula VI 
where each R is the same or different and is selected from the group consisting of optionally substituted alkyl; optionally substituted, saturated, unsaturated or aromatic carbocyclic rings; optionally substituted, saturated, unsaturated or aromatic heterocyclic rings; optionally substituted alkylthio, optionally substituted arylthio; optionally substituted alkoxy; optionally substituted dialkylamino; organometallic species; and polymer chains; and where the corresponding radical, R., is a free radical leaving group;
with (i) in the case of n=0,
a free radical of formula R., or
(ii) in the case of n greater than 0,
a free radical of formula R. and
a monomer of Formula III.
As shown below free radical addition to compound 6 will generate an adduct 9 which can fragment to give a compound 10 (Formula I where Z=Rxe2x80x3CH2xe2x80x94)
Rxe2x80x2.+
In these examples both of the groups R and Rxe2x80x2 must be a free radical leaving groups.
Benefits of the process according to the present invention including the following:
a) low polydispersity polymers can be synthesised.
b) molecular weights increase in a predictable and linear manner with conversion which is controlled by stoichiometry.
c) the process can be used to provide a variety of low polydispersity polymers.
d) the process is compatible with a wide range of monomers and reaction conditions.
e) dithioesters of complex structures may be readily synthesised.
A detailed discussion of the benefits of low polydispersity and the other advantages referred to above is provided in WO 98/01478, including a description of how reactants and conditions can be adjusted to provide desired results.